Doctor Henglaar, a Tellarite, is the USS Excelsior's (and later, the USS Enterprise's) Chief Medical Officer. He served under Captain Ian Quincy Knapp aboard the USS Devonshire during the Dominion War. He first met one of his few close friends, then-Lieutenant Myra Elbrey, when an Away Team lead by Lieutenant Commander Rodriguez, and consisting of Lieutenant Brickey and Dr. Henglaar rescued Elbrey from the derelict USS Rutledge. Henglaar has the distinction of being one of the few people to kamikaze a starship into a Grey warship and live.

With a typical Tellarite personality, he is diplomatically described as "no-nonsense" and less flatteringly as "curmudgeonly." He plays a mean hand of poker, in the sense that when he loses (frequently), he gets mean. Unskilled as a fighter, he tends to lose his fights, and is now banned from most of the gambling facilities and all of the bars on Deep Space 12.

Because of his "bedside manner", his patients tend to be exceedingly healthy or very, very sick, the former so as to avoid having to go to Sickbay during his shift. (It has been rumored that a colleague has described Dr. Henglaar as having "a heart of radium. It's warm, but you still don't want to get too close.") After several years' exposure to the Ba'ku rings, Henglaar was cured of a relatively rare hormonal imbalance, resulting in a more mature (for a Tellarite) appearance.

Henglaar's mother is deceased. His brother Falo, (Star Trek: Hidden Frontier: "Addictions") is currently serving time for being a major supplier of a particularly potent form of the illegal Venus drug. Falo himself had become addicted to the same drug he sneeringly sold to his customers, whom he despised as weak and stupid. Henglaar's niece, Falo's daughter, Silan, is currently under Henglaar's guardianship. Henglaar was the one who arrested Falo after an elaborate sting operation, bringing to a satisfying end their rather violent sibling rivalry. Henglaar has been married and divorced several times, making him somewhat of an expert on what not to do in a relationship. He recently encountered Sou, a Tellarite engineer from his past and her Klingon husband, Qu'qul, with everyone surviving the experience mostly intact. Sou and Qu'qul both serve aboard the USS Independence, also stationed at Deep Space 12. (Star Trek: Hidden Frontier: "Entanglement")

Not many. Primarily consisting of Knapp and Elbrey, with Crewman Thalek having recently attached himself to Henglaar in what has been described as a case of "hero worship" after Dr. Henglaar saved Thalek's life. (Star Trek: Hidden Frontier: "Coward's Death") Henglaar had previously believed that he was relatively difficult for the Betazoid Elbrey to telepathically read, a notion she quickly disabused him of, although he still disputes some of her assertions about him. His friendship with Knapp is that of two soldiers who survived a war in the same foxhole together.